In the Muller-Rochow direct synthesis, methyl chloride is reacted with silicon in the presence of a copper catalyst and suitable promoters to form methylchlorosilanes, where in addition to a very high productivity (amount of silanes formed per unit time and amount of silicon used), a very high selectivity with respect to the target product dimethyldichlorosilane is also demanded. Dimethyldichlorosilane is required, for example, for preparing linear polysiloxanes.
Despite the high economic importance of the direct synthesis, some of its scientific background has not yet been studied. According to Lieske et al. in Silicone for Chemical Industry, Geiranger-Norway, Jun. 16-18, 1992, owing to the participation of three solids in the reaction, that is to say silicon, catalyst and promoters, the reproducibility of the experiment is frequently poor. In practical conditions, different batches of the direct synthesis proceed with different results despite identical material and reaction parameters.
The direct synthesis can be carried out batchwise or continuously; in industrial production only the continuous variant is used. The continuous direct synthesis is carried out in fluidized-bed reactors in which methyl chloride is used simultaneously as fluidizing medium and reactant. The silicon required is ground in advance to a powder of particle size from 20 to 700 .mu.m and mixed with copper catalyst and promoters to give the contact catalyst.
A continuous direct synthesis production campaign is begun with the induction phase. At the start of the induction phase, methyl chloride is introduced into the heated contact catalyst. Thereupon the start phase follows, in which the crude silane formation starts. The reaction initially proceeds with low selectivity and reactivity. The stable production phase is then reached. The production campaign ends when methyl chloride is no longer introduced into the contact catalyst.
When a reactor is operated continuously in a production campaign, after a substantially stable production phase, the production rate, based on methylchlorosilanes, and the selectivity, based on the target product dimethyldichlorosilane, decrease. Therefore, the production campaign must be ended after a certain time. A production campaign therefore usually lasts only a few days to several weeks.
The reactor, after completion of a production campaign, is drained, refilled with silicon, copper catalyst and promoters and brought back to reaction conditions. It can be readily seen that the cost-effectiveness of the direct synthesis can be increased by increasing the production rate and also by increasing the duration of the production campaign wherein selectivity remains substantially constant.
Activating the contact catalyst prior to the reaction with methyl chloride by a preliminary reaction with HCl is known. In U.S. Pat. No. 4,864,044 for example, a process is described in which silicon, copper catalyst with or without tin promoter, but in the absence of zinc promoters, can be activated by HCl at approximately 325 .degree. C. Among the numerous disadvantages of this type of activation are the following: zinc and zinc compounds may not be added until after the activation, since zinc, with HCl, forms zinc chloride which can readily sublime under the specified reaction conditions; the zinc will thus be removed from the contact catalyst during activation; activation can only proceed prior to the actual reaction with methyl chloride; a separate reactor is required for the activation; the reaction products from the activation, in particular trichlorosilane and tetrachlorosilane are unwanted byproducts of the methylchlorosilane synthesis; and the activation consumes from approximately 1 to 2% of the raw material silicon used.
The addition of HCl to the methyl chloride during the direct synthesis is disclosed, for example, by U.S. Pat. No. 4,966,986. This leads to a considerable change in the crude silane composition, the content of methyldichlorosilane in the crude silane being increased. However, this simultaneously considerably decreases the content of the desired dimethyldichlorosilane.